1. Field of the Invention
The present invention relates to a positive electrode for non-aqueous electrolyte battery such as a lithium ion secondary battery and to a non-aqueous electrolyte battery using the same.
2. Description of the Related Art
In view of characteristics of small size and light weight, a lithium ion battery is widely diffused as a drive power source of potable instruments. In recent years, its development for applications including not only power tools and assisted bicycles but HEV is also expected. Like this, properties required for the lithium ion battery are being divided into two opposites of high capacity and high output, and in all of the applications, the amount of heat generated in the inside of the battery increases steadily. In the lithium ion battery which is especially low in thermal stability as compared with other secondary batteries, the development of an elemental technology enabling the battery to make both battery performance and reliability compatible with each other under a high-temperature condition is a pressing need. In particular, in the high performance of recent years, in order to bring out the performance of a material to the limit, how to control a chemical reaction occurring other than insertion and extraction of an Li ion is an important factor as compared with the related-art development of elemental technology.
Concretely, there are damage due to heat generated at high-rate charge and discharge and damage under high-temperature charge storage. In particular, how to control a side reaction (oxidation reaction) between an electrolytic liquid and a positive electrode active material in the vicinity of a positive electrode in the charged state is important. A positive electrode active material such as lithium cobalt oxide is instable with respect to its crystal structure in the charged state, and a possibility that it reacts with an electrolytic liquid, etc. to cause disintegration of the crystal structure becomes high. Also, the electrolytic liquid is decomposed coupled with its high positive electrode oxidation action/catalytic action, and the battery performance is largely lowered by clogging of a separator and an increase of deposits on the negative electrode surface to be caused due to elution of decomposed products derived from the electrolytic liquid or elemental components derived from the positive electrode material.
In the case where the battery in the charged state is stored in a high-temperature state, as a method for improving properties, there is made an attempt to improve high-temperature storage properties by coating the surface of a positive electrode active material particle with an inorganic material, thereby directly suppressing a reaction between of the positive electrode active material and an electrolytic liquid (see JP-A-2001-143703 and JP-A-2003-221234). Also, there is made an attempt to improve high-temperature storage properties by coating the surface of a positive electrode with a conductive polymer, thereby lowering a reaction with an electrolytic liquid (see JP-T-2007-510267).
However, in the foregoing related-art technologies, coating of the surface of a positive electrode active material with an inorganic material involved a problem that the coating layer slips down due to expansion and shrinkage of the positive electrode active material following charge and discharge so that an effect for suppressing the reaction with an electrolytic liquid is lowered. Also, there were involved problems that it is difficult to form a coating layer uniformly and thinly and that a lowering of the charge-discharge performance is caused.
Also, coating of the surface of a positive electrode active material particle or the positive electrode surface with a polymer are favorable in view of points that the organic material is rich in flexibility and able to follow expansion and shrinkage of the active material and that a thin film is easily formed. However, there is involved a problem that in a battery of a total gel type, the charge-discharge performance is lowered due to a lowering of ionic conductivity. Also, in case of a locally partial gel, there is involved a problem that dissolution into the electrolytic liquid existing in a large amount within the battery so that the improvement effect is weakened with a lapse of time. These problems can be treated to some extent by controlling the molecular weight. To increase the molecular weight is not preferable in view of a manufacturing step because dissolution of the polymer component in a solvent is noticeably lowered, and handling properties are lowered, too. Also, when the molecular weight is too low, the foregoing problems become distinct, and furthermore, decomposition of the polymer per se is easy to occur, resulting in a new problem that the battery performance is deteriorated.
JP-A-2007-48462 discloses that a polymer to be used in the invention is used as a binder for preparing an electrode. However, any review on use of this polymer as an additive in a coating layer for coating the surface or an active material layer is not made therein.